They probably believe it is too rigid, although different mail clerks enforce it in different ways. Among the definitions making a piece non-machinable under Domestic Mail Manual (DMM) 101.1.2:

d. Contains items such as pens, pencils, or loose keys or coins that cause the thickness of the mailpiece to be uneven (see 601.2.3, Odd-Shaped Items in Paper Envelopes).

e. Is too rigid (does not bend easily when subjected to a transport belt tension of 40 pounds around an 11-inch diameter turn).